This invention relates to methods of correcting presbyopic vision, and more particularly to methods of correcting presbyopia by the utilisation of contact lenses.
Presbyopia is a condition of the ageing human eye wherein the eye loses its ability to focus sharply on objects throughout the normal range of vision, i.e. from distant to near objects. This condition is the result of a gradual hardening of the crystalline lens which occurs with age and prevents the ciliary muscle from completely adjusting the curvature of the lens, and thereby altering its focal length, as required for viewing distant or near objects. The condition is progressive, and cannot at present be corrected by any treatment of the eye itself. Accordingly, it is conventional to provide the patient suffering from presbyopia with bifocal corrective lenses, which may be either conventional spectacle lenses or contact lenses. In such lenses, usually one portion of the lens is adapted to provide the proper correction, if necessary, for sharp distant vision, while another portion of the lens is adapted to provide sharp near vision. In spectacles, for example, the person using the corrective lenses directs his vision through the distant vision portion or the near vision portion as necessary.
In the case of conventional spectacle lenses, the near vision zone is a segment generally located in the lower portion of the lens where the wearer may easily direct his/her vision through it for reading. In the case of a contact lens, it is also possible to have a near vision zone located in the lower portion of the lens. These are known as translating bifocal lenses.
In most attempts to provide patients with lenses to compensate for presbyopia, each eye has been provided with a bifocal lens, whether spectacle lens or contact lens, designed so that the eyes are used together, each eye viewing the object through the distant vision or near vision portion as appropriate. Thus each eye sees a focussed image of the object.
An alternative method of treating presbyopia has been used in recent years, wherein a contact lens focussed for near vision has been fitted to one eye and a contact lens focussed for distant vision to the other eye. This is commonly known as monovision.
With this arrangement, one eye presents a clear distant view (but an unclear near view) to the brain while the other presents a clear near view (but an unclear distant view). In many cases, the brain can select the desired image depending upon whether distant vision or near vision is desired, and ignore the unclear image. This arrangement has the advantage that the patient need not shift vision from one optic zone to another, which is especially troublesome in the case of translating bifocal contact lenses. However, it is believed that at most thirty percent of wearers fitted with monovision arrangements can tolerate a presbyopic vision correction of this type. In the remainder of the population, the presence of the blurred image from the eye not focussed on the intended object cannot be ignored and is so distracting that the general effect is unsatisfactory correction of the presbyopic condition.
Another method of overcoming the problems described above is fitting a presbyopic person having binocular vision with concentric bifocal contact lenses wherein one eye is provided with a contact lens having a distant vision central optic zone and a near vision peripheral optic zone, and the other eye is provided with a contact lens having a near vision central optic zone and a distant vision peripheral optic zone. The effect of these lenses is dependent on the size of the central optic zone in relation to the size of the pupil of the eye. Often this is made smaller than half the size of the normal pupil.
Presbyopic patients fitted with a pair of lenses consisting of one of each type find that they can see distant objects with both eyes but usually more clearly in the eye with the near centre lens. Near objects are also seen with both eyes, but usually more clearly in the eye with the distance centre lens. In general, there is binocular correction of both distance and near vision without the annoyance and confusion caused by the image in one eye being completely out of focus (as in monovision). This effect is achieved because each lens provides some correction for both distance and near objects, one eye being biased for distance (the near centre lens) and one eye being biased for near (the distance centre lens). Concentric bifocal lenses provide to each of the eyes two simultaneous images, one formed by the central zone and another by the peripheral zone.
The dominant image is that formed by the zone giving a larger coverage of the pupil area. For example, if the peripheral zone covers a larger area of the pupil than the central zone, the power of the peripheral zone will be dominant in forming the image. However, either the central or the peripheral zone could be constructed to cover a larger area. If the central zone was of larger area, the focus of the central area would be dominant. The eye with the lens which has the larger zone adapted for the visual task at hand is apparently selected by the brain, and the image formed in the other eye by the smaller zone of its contact lens assists in the visual task at hand.
This method effectively superimposes two images by having two focal lengths on the same axis. Each image is surrounded by the out of focus image of the other component. A problem in the wearing of such lenses is the fact that the human pupil size is not constant and varies from person to person. At small pupil sizes very little aperture is available for the distance image light and this image may be lost in the out of focus near image. At large pupil sizes the amount of light entering the eye for the near image light is very low and for reading, the near image will tend to be lost in the out of focus distance image. At less extreme pupil sizes i.e. not too large or too small, the two images will receive equal amounts of light thus making it possible to achieve good vision at both near and distance.
Another form of bifocal contact lens which is not pupil size dependent, and can be considered as a full aperture bifocal is the diffractive bifocal contact lens. In this type of lens, the whole aperture provides rays to both near and distance images. In such a lens, the diffractive power may be additive to (or subtractive from) the basic refractive power of the lens. Thus a lens may have a basic power e.g. for far vision, provided by the shape, curvature and material of the lens, and a different power e.g. a greater power for near vision provided by diffractive means on the lens so as to give a diffractive power which adds to the basic refractive power of the lens. One way of providing diffractive power is by means of a zone plate.
A bifocal lens can also be formed in which the basic refractive power is provided for near vision, and the diffractive means is provided to give negative power to the lens and effectively subsract power from the basic power of the lens to provide for distance vision. A diffractive lens nullifies the effect of pupil size and enables the lens to be designed so that the available light is divided so that about half the light goes into each image. The ratio of intensity between the near and far images may be chosen independently of the add power, pupil size or base power. Diffractive lenses as available in the marketplace are designed so that in terms of the ratio of intensity between the near and far images are substantially 0.5:0.5 in both eyes in contrast to the so-called monovision condition which is essentially 0:1 in one eye, and 1:0 in the other eye. The concentric bifocal contact lens as indicated above provides to each of the eyes two simultaneous images one formed by the central zone and the other by the peripheral zone. The dominant image is that formed by the zone giving a larger coverage of pupil area. Thus the concentric bifocal is pupil size dependent although it does overcome some of the problems associated with monvision contact lenses.